1. Field of the Invention
The present invention relates generally to a paper conveying device for conveying paper such as a document, and more particularly, to an automatic paper conveying device mounted on an electrophotographic copying machine or the like for automatically setting paper such as a document in a predetermined position.
2. Description of the Related Art
An automatic document conveying device mounted on an electrophotographic copying machine, a facsimile or the like for conveying a document (paper) so as to automatically set the document in a predetermined position has been known.
Many of conventional automatic document conveying devices include a mechanism for conveying a document by a conveying belt. In this type of automatic document conveying device, control is so carried out that when the document is set, the front end or the rear end of the document is stopped along a predetermined reference line.
One example of methods of control conventionally carried out so as to exactly set the end of a document along a reference line is a system of overrunning once the end of a document by a predetermined amount from the position where the document is to be set and then, driving a conveying belt in the reverse direction to switch the document back so that the end of the document collides with a projection provided in the position of the reference line. In such a system of switching the document back, control for overrunning the document and switching the document back has been conventionally managed by time.
Furthermore, there has been a method of rotating a motor for driving a conveying belt in the reverse direction although the motor is not completely stopped at the time of overrunning a document once and then, driving the conveying belt in the reverse direction.
Meanwhile, the above described system of switching the document back so that the end of the document collides with the projection provided in the position of the reference line has the disadvantage in that the stress is applied to the document at the time of the collision, so that the document is liable to be, for example, wrinkled or folded. In particular, the amount of overrun and the amount of switchback of the document have been conventionally managed by time as described above, so that sufficient time to manage the amounts must be set. Therefore, the system has the disadvantage in that the stress applied to the document is increased.
Furthermore, the document is overrun by a distance greater than necessary and switched back. Therefore, the system also has the disadvantage in that it takes long to set the document and it also takes long to replace the document with another document.
On the other hand, in the method of rotating the motor for driving a conveying belt in the reverse direction before the motor is completely stopped at the time of switching the document back, time required for control may be short. Since a large force is exerted on the conveying belt, however, a large frictional force is produced between the conveying belt and the document and slip occurs therebetween. Therefore, the method has several disadvantages. For example, the document is damaged, the alignment precision of the document is degraded, or a static charge is produced.